This is an application for a Mentored Research Scientist Award, to support the candidate while she develops an independent research program focused on the molecular mechanisms that regulate the human cell division cycle. The candidate will pursue an intensive program of investigation at the bench; participate in research conferences and journal clubs at the sponsoring institution devoted to the cell cycle, cancer, and gastrointestinal diseases; and learn the technique of cellular microinjection. Cycli dependent kinase 1 (Cdk1) is known to initiate mitosis, and the closely related enzyme Cdk2 is known to be required for initiation of DNA synthesis. Using stable cell clones with inducible expression of an inactive Cdk2 mutant that disrupts the function of the normal enzyme, the candidate has obtained evidence for an essential function for Cdk2 that links DNA synthesis with mitosis. Inhibition of Cdk2 at this point arrests cells with replicate DNA, and Cdk1 accumulates in an inhibited, tyrosine-phosphorylated form. This step appears to be a significant point of regulation of the normal epithelial cell cycle by transforming growth factor-6 and by checkpoint pathways that monitor genotoxic stress and suppress neoplasia. The candidate will examine how expression of the inactive Cdk2 mutant alters the activity of known key regulators of mitotic entry. The activity of the affected regulators will be manipulated, to assess whether their function(s) is rate limiting when Cdk2 activity is impaired. The mechanism by which Cdk2 alters the activity of the regulator will be investigated in detail. This work will open a new, highly productive avenue of investigation of the mammalian cell cycle and provide the candidate with the research program, knowledge base, and skills she will need for an independent research career.